1. Technical Field
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that processes a toner image with heat and pressure on a recording medium, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features, which incorporates such a fixing capability.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and setting the toner with heat and pressure.
A modern image forming apparatus accommodates a wide range of printing applications using various types of recording media upon which toner images are printed and fixed, which require different levels of image quality, particularly, in terms of glossiness and uniformity of gloss across a toner image. For example, printing of photographs and computer-generated images, which typically contain multiple colors, necessitates a higher level of image quality and gloss uniformity than is required for a monochrome image as in a text-only print.
Various techniques have been developed to provide a printing process that can print a toner image with a uniform, high gloss appearance. Some of these techniques employ a special type of toner that is transparent in color, typically called “clear toner”, to create transparent, glossy effects on those areas of a recording medium where any color toner is not present. Other techniques address duplex printing of toner images with uniformly high gloss on both sides of a resulting print.
For example, one such technique proposes a fixing device that includes a thermal pre-fixing unit and an image gloss adjustment unit comprising a smooth, endless rotary belt on which a recording medium is conveyed after processing through the pre-fixing unit.
In this fixing device, the pre-fixing unit initially processes an unfixed, powder toner image on a recording medium through heating, which melts toner particles into a semi-fluid, soft pliable state. After pre-fixing, the recording medium is then conveyed on the endless rotary belt with the toner image pressed against the smooth moving surface of the belt, which imparts a certain amount of gloss on the toner image as the molten toner gradually cools and solidifies while conforming to the smooth surface of the belt. The image gloss adjustment unit adjusts the glossiness of the toner image by adjusting a distance or duration during which the toner image travels on the belt while subjected to pressure.
Another technique proposes a belt-based fixing device that includes an endless fixing belt looped for rotation around multiple rollers, including a pair of first and second motor-driven rollers, as well as a cooler disposed between the first roller and another belt-supporting roller inside the loop of the fixing belt.
In this fixing device, the first motor-driven roller is driven at a rotational speed slower than that of the second motor-driven roller, so as to tighten the belt between the first motor-driven roller and the cooler downstream in a longitudinal direction in which the belt rotates around the multiple rollers. Provision of the belt tightening mechanism prevents the fixing belt from deformation due to being held on the supporting rollers for extended periods of time, as well as sags and creases resulting from thermal expansion/contraction of the belt subjected to repeated heating and cooling cycles, which would otherwise affect uniformity of gloss across a resulting image.
Although generally effective for its intended purpose, the belt-based fixing device depicted above has several drawbacks. That is, tightening the belt in the longitudinal direction to establish close contact between the belt and the cooler increases the torque required to drive the belt. Also, such arrangement necessitates an extremely high level of precision during assembly of the fixing device. Further, increased longitudinal tension in the belt can lead to accelerated wear and tear of the adjoining surfaces of the belt and the cooler, which slide against each other during rotation of the belt.